Knowing your sweat rate lets you plan how much to drink on long runs, rides, and hot-weather training so you neither dehydrate nor overdrink. Weigh yourself before and after a workout, add the fluid you drank, and this calculator works out your total sweat loss and hourly rate.
How it works
The estimate rests on the fact that 1 litre of body fluid weighs about 1 kg, so weight change tracks fluid balance:
- Sweat lost (L) = (pre-weight − post-weight) + fluid drunk − urine passed
- Sweat rate (L/hr) = sweat lost ÷ (duration in minutes ÷ 60)
- Rehydration target (L) = sweat lost × 1.5 (ACSM guidance, ~150%)
Fluids drunk are added back because they masked part of the loss; urine is subtracted because it was not sweat.
Example
A runner: pre 70.0 kg, post 68.8 kg, drank 0.5 L, no urine, 60 min:
- Sweat lost = (70.0 − 68.8) + 0.5 − 0 = 1.2 + 0.5 = 1.7 L
- Sweat rate = 1.7 ÷ 1.0 hr = 1.7 L/hr
- Rehydrate with 1.7 × 1.5 ≈ 2.55 L over the following hours
| Quantity | Value |
|---|---|
| Sweat lost | 1.7 L (1700 ml) |
| Sweat rate | 1.7 L/hr |
| Rehydration target | ~2.55 L |
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